On July 17, California’s State Assembly and Senate voted to expand and extend[2] through 2030 the state’s pioneering cap-and-trade[3] program to curb greenhouse-gas[4] emissions. It was a huge victory for Governor Jerry Brown, who had insisted that both houses approve the measure by a two-thirds majority to protect it from legal challenges.

Brown’s triumph, which he signed into law[5] on July 25, reinforced California’s status as the United States’ leader in beginning the transition to a post-carbon economy. The governor has insisted that far-sighted leadership in Sacramento can fill the void created by President Donald Trump’s disastrous withdrawal from the Paris climate agreement[6].

But can the wealthiest and most populous U.S. state conduct its own foreign policy within the U.S. constitutional system? Writing in Foreign Affairs in 1993, the journalist James O. Goldsborough pondered[7] whether California should do so. A quarter of a century later, the verdict is in. As the United States has receded from global climate leadership, California has filled the diplomatic breach.

ALL THE WORLD'S ITS STAGE

Since last year’s presidential election, California has emerged as a focal point for resistance[8] to the Trump administration’s policies. This is hardly surprising. Trump lost the state by a wide margin, garnering less than one-third of the popular vote in the general election, and his opponent, Hillary Clinton, bested him by 30 percentage points—the largest victory for a Democrat in the state since 1936. And yet on election night, California’s progressive voters discovered just how far removed they were from the national mood. Trump won the electoral college after running on a nativist, protectionist, and anti-science platform at odds with most of California’s ethnic diversity, commitment to open trade[9], and embrace of environmentalism. The stage was set for confrontation.

MIKE BLAKE / REUTERS
Solar panels on the roof of a building in Los Angeles, June 2017.

Since then, Brown has staked out progressive positions starkly at odds with the White House’s views on immigration, trade, and climate change. On January 24, four days after Trump’s dark inaugural address[10] in Washington, Brown